1. Field of the Invention
The present invention relates to unbonded PC (prestress concrete) steel strands having a plastic sheath and used for posttensioning concrete structures, and to a process for making such unbonded PC steel strand.
2. Related Art Statement
In general, unbonded PC steel strands having a plastic sheath have been used for preventing the PC steel strands from bonding to concrete in posttensioning of concrete structures in which the PC steel strands placed in concrete are stretched to produce stress in the concrete structures after hardening of concrete.
Conventional unbonded PC steel strands have a corrosion inhibitor such as a greaselike filler between the PC steel strands and their plastic sheaths for improving corrosion protection and lubricity of the PC steel strands as described, for instance, in Japanese Utility Model Application Publication No. 1,160/67, in which the PC steel strand is coated with a filler such as grease or vaseline having fluidity and then helically wrapped with paper tapes and further continuously covered with a synthetic resin sheath having a certain thickness over the whole length of the steel stranded wire, or in U.S. Pat. No. 3,646,748, in which the PC steel strand is coated with a greaselike corrosion inhibitor of a predetermined thickness, and then covered with a flexible synthetic resin sheath.
The prior unbonded PC steel strands filled with greaselike filler between PC steel strands and their sheaths, however, have many problems: The filler penetrates into gaps between stranded wires during the coating operation, increasing the amount of the filler expended in production. Grease adhering to both ends of the PC steel strands must be wiped off at an expenditure of time and labor in order to positively anchor both ends of the PC steel strands in use, and, because complete removal of grease is extremely difficult, positive anchoring of the ends of the PC steel strand cannot be obtained. Furthermore, if the PC steel strands are obliquely or curvedly tensioned, tensile stress of the strands decreases in accordance with increase in the distance from the anchored end, and the decreasing degree of this tensile stress naturally varies in accordance with curvature and angular change of the standard wire. A serious disadvantage arises due to a different coefficient of friction of the PC steel strand to a sheath wall, particularly because of non-uniform distribution of the filler in the space between the wires of the strand and the sheath. Therefore, the problem arises that it is difficult to ensure uniform and complete tension over long periods of time.
Further, there is a problem that the anchored portions are apt to rust as a result of wiping off grease from both anchored end portions of the PC steel strand.
The above U.S. Patent specification also discloses application of a thin coating of a polymer selected from the group consisting of tetrafluoroethylene and copolymers of tetrafluoroethylene with 5 to 35% hexafluoropropylene to the outer surface of the steel strand. Application of such Teflon synthetic resin coating can reduce consumption of the greaselike filler and solve the problem of wiping off grease, but there remains a problem that the greaselike filler does not enter between wires of the PC steel strand and corrosion protection of the PC steel strand wire by the greaselike filler cannot be obtained.